json/doc/mkdocs/docs/api/basic_json/get.md

basic_json::get

// (1)
template<typename ValueType>
ValueType get() const noexcept(
    noexcept(JSONSerializer<ValueType>::from_json(
        std::declval<const basic_json_t&>(), std::declval<ValueType&>())));

// (2)
template<typename BasicJsonType>
BasicJsonType get() const;

// (3)
template<typename PointerType>
PointerType get_ptr();

template<typename PointerType>
constexpr const PointerType get_ptr() const noexcept;

1. Explicit type conversion between the JSON value and a compatible value which is CopyConstructible and DefaultConstructible. The value is converted by calling the json_serializer<ValueType> from_json() method.

   The function is equivalent to executing

   ValueType ret;
   JSONSerializer<ValueType>::from_json(*this, ret);
   return ret;
   

   This overloads is chosen if:

   • ValueType is not basic_json,
   • json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&), and
   • json_serializer<ValueType> does not have a from_json() method of the form ValueType from_json(const basic_json&)

   If the type is not CopyConstructible and not DefaultConstructible, the value is converted by calling the json_serializer<ValueType> from_json() method.

   The function is then equivalent to executing

   return JSONSerializer<ValueTypeCV>::from_json(*this);
   

   This overloads is chosen if:

   • ValueType is not basic_json and
   • json_serializer<ValueType> has a from_json() method of the form ValueType from_json(const basic_json&)

   If json_serializer<ValueType> has both overloads of from_json(), the latter one is chosen.

2. Overload for basic_json specializations. The function is equivalent to executing

   return *this;
   

3. Explicit pointer access to the internally stored JSON value. No copies are made.

Template parameters

ValueType

the value type to return

BasicJsonType

a specialization of basic_json

PointerType

pointer type; must be a pointer to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Other types will not compile.

Return value

1. copy of the JSON value, converted to ValueType
2. a copy of #!cpp *this, converted into BasicJsonType
3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; #!cpp nullptr otherwise

Exceptions

Depends on what json_serializer<ValueType> from_json() method throws

Notes

!!! warning

Writing data to the pointee (overload 3) of the result yields an undefined state.

Example

??? example

The example below shows several conversions from JSON values
to other types. There a few things to note: (1) Floating-point numbers can
be converted to integers, (2) A JSON array can be converted to a standard
`std::vector<short>`, (3) A JSON object can be converted to C++
associative containers such as `std::unordered_map<std::string, json>`.
    
```cpp
--8<-- "examples/get__ValueType_const.cpp"
```

Output:

```json
--8<-- "examples/get__ValueType_const.output"
```

??? example

The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
conversions are made and a `#cpp nullptr` is returned if the value and the requested pointer type does not match.
    
```cpp
--8<-- "examples/get__PointerType.cpp"
```

Output:

```json
--8<-- "examples/get__PointerType.output"
```

Version history

1. Since version 2.1.0.
2. Since version 2.1.0. Extended to work with other specializations of basic_json in version 3.2.0.
3. Since version 1.0.0.